1. Field of the Invention
The present invention generally relates to optical correction of charged particle beam tools and, more particularly, to reduction and compensation of aberrations in electron beam (e-beam) projection systems.
2. Description of the Prior Art
Numerous techniques are known utilizing charged particle beams and are in widespread use for manufacture of integrated circuit devices, in particular. For example, charged particle beams are used for implantation of impurities, inspection (e.g. with scanning electron microscopes) of structures for process evaluation and development and for lithographic patterning of substrates and layers deposited thereon.
Essentially, lithography processes define potentially very minute areas and shapes on a surface through selective exposure and removal of portions of a layer of resist to expose areas of the surface for further processing by, for example, etching, implantation and/or deposition. In general, for semiconductor device manufacturing, such exposures of the resist have predominantly been made with electromagnetic radiation (EMR) rather than with charged particle beams.
However, there is a strong incentive in the manufacture of integrated circuits to increase integration density to the greatest possible degree consistent with acceptable manufacturing yields. Device arrays of increased density provide increased performance since signal propagation time is reduced and noise immunity is increased with reduced connection length and capacitance. Further, increased device density on a chip allows greater chip functionality as well as greater numbers of devices which can be manufactured on a chip of a given area. This, in turn, results in increased economy of manufacture if manufacturing yields can be maintained.
Device size and density is a function of the minimum feature size which can be reliably produced in the course of patterning the resist. Minimum feature size is limited by the resolution of the exposure which, in the case of EMR, is essentially determined by the wavelength of the radiation utilized to expose the resist. Wavelengths corresponding to deep ultra-violet (DUV) is used almost exclusively for current integrated circuit manufacturing processes and can produce minimum feature sizes as small as 0.25 microns. While use of DUV lithography (DUVL) may be extended to minimum feature sizes of about 0.13 microns (130 nanometers) it is generally considered that other lithographic exposure techniques such as extreme ultra-violet lithography (EUVL), x-ray lithography (XRL), and charged particle lithography (ion beam projection lithography (IPL) and electron beam projection lithography (EBPL)) will be required at smaller feature sizes.
Among these techniques, electron beam projection lithography has the advantage that electrons can readily be controlled and manipulated by electromagnetic fields acting as lenses, deflectors and correctors. Electron beam projection lithography is also able to produce a plurality of pattern elements or pixels in a single exposure. However, it can be appreciated that e-beam exposure is only viable as an exposure medium for high volume production of integrated circuits (ICs) if a throughput comparable to EMR exposure techniques such as DUVL can be realized. To date, EBPL tools have employed beams which are limited in transverse dimensions to about five microns and therefore contain only limited numbers of pattern elements or pixels, generally on the order of about one thousand pixels or less per exposure. These systems are often referred to (collectively with individual pixel exposure systems) as probe forming systems.
Current IC chip designs, however, include on the order of one billion pixels. This number may increase by a factor of ten to one hundred or more in the foreseeable future. Therefore, a very large number of sequential exposures is required to make an exposure of a complete chip area pattern and throughput of probe-forming systems is unacceptably low. This problem cannot realistically be approached by reduction of shot exposure time in view of the large number of exposures which must still be made.
Therefore, any practical solution must increase the number of pixels which can be exposed simultaneously, in parallel. Ultimately, it would be desirable to expose an entire chip area pattern with a single exposure. However, exposure of such an extensive area is not currently practical for various reasons including distortion and field curvature of the electron-optical system (which are correctable to tolerable residual error over only small areas) and available flatness of a wafer surface over a chip area. Lack of wafer flatness can derive from both wafer manufacture and/or mounting of the wafer in the e-beam exposure tool and is collectively referred to as target height variation. That is, among other practical considerations, focus must be adjusted within sub-fields of chip areas to compensate for field curvature and distortion as well as irregularities of topography of the wafer or other structures formed thereon so that resolution of features of a size that would require e-beam exposure, in the first instance, can be achieved.
A solution which is currently feasible, however, is to project a portion of a chip pattern or sub-field which, while small relative to chip area (e.g. 0.01% of the chip pattern), is large compared to a pixel in probe-forming systems. Generally systems capable of projecting increased numbers of pixels also employ optical reduction of projected image size from a sub-field pattern provided on a reticle, employing what is referred to hereinafter as large area reduction projection optics (LARPO). However, such systems have not heretofore been successfully applied to minimum feature sizes approaching or below one-tenth micron and in which the pattern may include one million pixels or more.
The consequences of extending this approach to smaller minimum feature size regimes include a need to project a sub-field having dimensions of about several tenths of a millimeter on a side as flawlessly as required for pattern fidelity commensurate with ground rules of 100 nm and smaller (e.g. a dimensional tolerance of the image of about 10% to 15% of minimum feature size). Further, in order to cover a chip area which may measure several centimeters on a side, the positioning and shape of the image must be of comparable accuracy and fidelity and achieved at very high speed. This latter requirement further implies that the beam must be deflected off the central axis of the beam generating particle-optic system.
In this regard, those skilled in the art will recognize that a projected electron beam will include imperfections or geometric aberrations of many types and that the number of types of aberrations and their size will increase when the beam is deflected off-axis. Fortunately, some of these aberrations can be corrected dynamically in accordance with beam position by appropriate driving of lenses and correctors in synchronism with deflectors.
In probe-forming systems, there are only two dynamically correctable aberrations: astigmatism and field curvature. These aberrations are respectively correctable with one dual axis quadrupole stigmator and one focusing device forming a correction lens. (The latter is often referred to as a focus coil since it is generally small and has much less inductance than the major lenses of the system. The major lenses of the system cannot, as a practical matter, be dynamically driven for that reason.) Development of appropriate corrections for probe-forming systems is well-known, as is the fact that the alteration of focus will cause both a change in image magnification and rotation of the image.
The effects of magnification change and rotation are, however, negligible for beam sizes (corresponding to a much larger sub-field in LARPO systems) characteristic of probe-forming systems. That is, the beam size and orientation error which results from magnification change or rotation over the small beam transverse dimension will be a tolerably small fraction of the beam size. Further, for such beam sizes, astigmatism affecting shape of a feature within the beam (referred to as feature shape distortion (FSD) in LARPO systems) and astigmatism affecting the shape of the beam (referred to as sub-field distortion (SFD) in LARPO systems) are indistinguishable.
The same is not true for a LARPO system in which correction is substantially more complicated. Specifically, the projected sub-field is much larger than the beam in a probe-forming system and in comparison with minimum feature and pixel size. Therefore, sub-field distortion and feature distortion must be separately compensated since they derive from different sources and locations in the electron-optical column. Further, correction of either type of distortion may affect the magnitude of the other. Similarly, changes in magnification and rotation of the image cannot be ignored when the sub-field size is much larger than the feature size. In LARPO systems utilizing curvilinear variable axis lenses (CVAL), as described in U.S. Pat. No. 5,635,719, which is hereby fully incorporated by reference, additional complexity arises from the superposition of focusing and deflection fields leading to aberrations referred to as "hybrid".
It has been demonstrated, in theory, as described in "a Multistage Deflector Theory" by Hosokawa et al. (Optik 58 (1981), p. 241) that in combination with deflectors, two dual-axis quadrupole stigmators and three focus elements are necessary and sufficient to compensate regular and hybrid astigmatism and field curvature. However, while such a minimum of optical elements may be sufficient, in principle, to achieve such corrections, no practical embodiment is suggested by Hosokawa et al. Moreover, while it is known that single correctors of probe-forming systems must be placed in particular locations to avoid causing additional errors, it is generally considered by those skilled in the art that the placement of multiple focus and astigmatism correction elements is, in theory, not critical in charged particle beam systems since the magnetic field strengths can be adjusted relatively readily by control of the energization currents. Therefore, accepted theory of charged particle optics provides no guidance concerning the location of multiple astigmatism and focus correction elements.
Nevertheless, while it may be accepted theory that two stigmators would be necessary to compensate for the differences in feature shape distortion and sub-field distortion when the two forms of distortion due to astigmatism can be distinguished, it is equally clear that the two stigmators (or the three focus elements) cannot be placed at the same axial location (or closely adjacent each other) without their effects being superimposed to effectively function as fewer elements than theorized to be required. It is also generally accepted, as alluded to above, that correction of sub-field distortion (SFD) and correction of feature shape distortion (FSD) are interrelated in a manner which may be largely unpredictable based on relative stigmator location. Hosokawa et al. provides no guidance in regard to placement or relative energization of such two stigmators in order to simultaneously correct both forms of distortion.
Such high levels of performance of charged particle beam systems as is currently desired from e-beam projection lithography systems, particularly of the LARPO and/or CVAL tppe, has not heretofore been required for commercial purposes or, for that matter, been readily observable. Charged particle projection systems having beams of sufficient size to require such performance have only recently been developed. No prior effort to develop a practical application of the theory of Hosokawa et al. or a working embodiment in accordance therewith is currently known to the inventors. However, it should be recognized that there has been no practical solution to correction of both feature shape and sub-field distortion and/or correction of field curvature while minimizing both image rotation and magnification change, even though practical electron and ion beam devices, the theory of operation and electromagnetic beam control therein and the interaction of correction of types of astigmatism and field curvature has been known for many years.